1. Field of the Invention
The present invention relates to an improvement in a process for fixing a toner image obtained by the electrophotographic technique, electrostatic printing and the like, and more particularly it is concerned with an improvement in the prevention of paper-winding and the offset phenomenon, which have caused problems at the time of fixing the toner image by means of a heat roller.
2. Description of the Prior Art
It has been heretofore known that an electrostatic latent image formed on a photoconductive layer or an insulating layer is developed by using charged fine particles, that is, a toner and the thus obtained toner image is fixed to produce a copy, and it has also been known that after development of the electrostatic latent image, the toner image is transferred to a transfer material and fixed to produce a copy.
As a process for fixing the toner image, various processes have been known, for example, a process of heating and melting the toner by means of a heater, etc. to cause the melted toner to adhere to a support and solidify it, a process of softening or dissolving the binder resin of the toner with an organic solvent to fix the toner on a support by the evaporation of the organic solvent, and the like. Further, another process also has been known, in which a toner image obtained on a support is caused to pass between heat rollers so that the toner is melted, solidified and fixed by the heat and pressure. Such a fixing process using a heat roller is disclosed in, for example, U.S. Pat. Nos. 3,108,863, 3,612,820, 3,649,992, 3,743,403 and 3,751,216. The abovementioned prior art fixing process using a heat roller has many advantages e.g., the toner image is directly brought into contact with the heated region of the heat roller so that the toner is melted at a very high efficiency and thereby the fixing efficiency and fixing rate are improved resulting in reduction of the electric power consumed, a small size apparatus and prevention of fire. However, the prior art fixing process is disadvantageous in that the image quality is deteriorated on account of the paper-winding and offset phenomenon. The expression "paper-winding" as used herein means that the paper adheres to and winds around the heat roller, and "offset phenomenon" means that a part of the toner melts and adheres to the heat roller.
For eliminating such disadvantages, heretofore, Teflon, silicone and the like have been applied to the surface of the heat roller to prevent the paper-winding and to avoid the offset phenomenon. This counterplan is considered effective to some extent, but it is not completely satisfactory.
It has been found from the various studies of the present inventors that when the following relationship is satisfied: EQU F.sub.TR &lt; F .about. F.sub.TP
wherein F.sub.TR is the interfacial tension between the heat roller and the toner, F is the cohesion between toner particles and F.sub.TP is the interfacial tension between the toner and the paper, no paper adheres and winds around the heat roller, nor does the offset phenomenon occur so that the toner image can be effectively fixed.
The factors determining F.sub.TR are mainly the surface free energy of the heat roller and that of the toner. In order to effectively prevent a paper from adhering and winding around the heat roller and avoid the offset phenomenon, the surface free energy of the heat roller should be lowered as far as possible and that of the toner should be made higher. In this case, if the surface free energy of the toner is higher than that of the paper, the toner hardly fixes on the paper. The surface free energy of the toner should be appropriately selected. Further, a high molecular material which is a main constituent of the toner is generally a material of a low surface free energy. Among such high molecular materials, even the material of a higher surface free energy exhibits only a surface free energy lower than that of water. Consequently, the range of the surface free energy of the toner is also restricted in view of the material.
Teflon, silicone, etc., which have been applied to the surface of a heat roller as a releasing agent, have a critical surface tension of 17 to 18 dyne/cm, and polystyrene, polymethyl methacrylate, etc. of a relatively high surface free energy which are considered suitable as the binder resin of the toner for the process for fixing the toner image using a heat roller have a critical surface tension of 34 to 37 dyne/cm. From this fact, it has been expected at first that no paper would adhere and wind around the hot roller, nor would the offset phenomenon occur as mentioned above. In practice, however, the study of the present inventors has showed that the binder resin of the toner is melted within the range of the temperature at which the toner is sufficiently fixed on the paper and the surface free energy of the toner is extremely lowered so that the paper adheres to and winds around the heat roller. In other words, it has been found that the cohesion of the thus melted toner is extremely decreased as compared with the solid state thereof and that even when a material of a relatively large critical surface tension is used as the binder resin of the toner, the paper adheres to and winds around the heat roller and the offset phenomenon takes place.
As a result of the study of various binder resins for the purpose of preventing the cohesion of the toner from decreasing in the melted state, it has been found that a rubber-like material usually exhibits a cohesion lower than that of a resin, but within the range of the fixing temperature onto a paper, etc., its cohesion is larger than that of a resin. This finding further has showed that when a rubber-like material is used as the binder resin of a toner, the lowering of the cohesion of the toner in the melted state can be restrained, and the paper-winding and the offset phenomenon which are liable to take place at the time of fixing the toner image using the heat roller can be prevented.
Generally, the binder resin of a toner should be that capable of giving a toner having the following properties: (1) it should be excellent in its powdering property, (2) it should be completely fixed on a support for the toner image such as paper, film and the like at a temperature at which the support is not scorched, (3) it should be excellent in its triboelectric property, (4) it should exhibit good transferability in the transferring type process, and the like. However, the above-mentioned rubber-like material is not satisfactory in these properties required for the binder resin of a toner. In particular, the rubber-like material is extremely poor in powdering property since it exhibits a rubber-like elasticity at a normal temperature, and it is very difficult to fix the rubber-like material on a paper, film and the like.
A process for fixing a toner image according to the present invention has been developed on the basis of the view of the present inventors that when a heat roller is used to fix a toner image, the toner to be used should be a particular toner which is different from that having been used in the conventional fixing process.